Solar cell with discontinuous junction

ABSTRACT

A silicon solar energy cell having a substantially constant voltage despite significant increases in illumination, which cell has a back surface junction that is discontinuous and has spaced, shorted portions formed therein.

In the manufacture of silicon solar energy cells, an impurity isdiffused into a wafer of silicon that has previously been doped with animpurity of opposite polarity. Thus, for n-p type silicon solar cells,the diffusion process usually employs compositions of phosphorus orarsenic where the silicon wafer that is the precursor of the cell hasbeen doped with, e.g., boron. After an electrical junction has beenestablished in the wafer, contacts are applied to the front surface ofthe cell, which is adapted to absorb light impinging thereon and therebygenerate an electrical current, and to the back, non-exposed surface ofthe wafer. Since diffusion normally takes place before contacts areapplied, it is necessary either to protect the back surface of thesilicon wafer during diffusion or later to eradicate the undesirablejunction that has been formed at the back surface.

An important solution to the problem of making unnecessary theprotection of the back surface of the silicon wafer during diffusion isdisclosed in my U.S. Pat. No. 3,990,097, issued Nov. 2, 1976. Adivisional application having the same disclosure as said patent ispresently pending under Application Ser. No. 705,063, which divisionalapplication has a filing date of July 14, 1976. In such patent andapplication, a method and product are disclosed whereby the back surfaceof the wafer is not protected during diffusion, but metallic aluminum isapplied over the layer of diffusant glass formed on the back surfaceafter diffusion. This coating of aluminum is then alloyed through theglass and reverses the polarity of the diffusant junction formedinwardly of the back surface of the wafer. Further, the aluminum coatingmay be used as the back contact for the cell, itself, and an aluminumback contact has been found most useful in silicon solar energy cells.

A specific application has now arisen with respect to an aluminum orother metallic coating alloyed on the back surface of a silicon wafer,whether or not that layer has been applied to a layer of diffusant glassin accordance with the method and product of my previous patent andapplication, or directly to the back surface of the wafer, which backsurface has been protected during diffusion or from which the diffusantglass formed on such back surface has been removed. The phenomenon hasbecome particularly evident with respect to certain uses to which solarcells have found employment. One such use, for example, is to rechargebatteries. In many of the applications to which solar cells have beenput, an important utilization is to continuously apply a "tricklecharge" to a battery that powers a device located far from moreconventional sources of electricity. Thus, solar panels using amultiplicity of cells have found employment on oil and gas drillingrigs, and for recharging battery-powered microwave relay stationslocated at the peak of mountains.

Whether the battery being recharged is small or large, the possibilityof overcharging the battery can present a serious hazard. While it ispossible to use a voltage regulator to prevent overcharging, such aregulator increases expenses, may reduce efficiency of the overallsystem, and is simply another piece of equipment subject to corrosionand ultimately, to failure. Without the voltage regulator, the batterycan be overcharged to the point of failure, indeed even to explosion. Inany case, failure of the battery leads to inoperativeness of the loaddevices, e.g., the light on a buoy, or the operation of a solar poweredwatch or flashlight or pocket calculator.

In terrestrial uses of solar cells, it will, therefore, be apparent thatit is widely desirable for a solar cell to maintain a substantiallyuniform voltage, since there will be wide variations in illuminationpresent in ambient conditions. In outdoor conditions, cloud coverage andreflective surfaces present will vary by hundreds of times. Indoors,there will be similar variations from low light levels up to themultiple suns provided by a flashlight.

It is, consequently, a primary object of the present invention toprovide a solar energy cell in which, once the desired voltage has beenreached, exposure to significantly increased illumination will notsignificantly increase the voltage generated by the cell to a level suchthat a device or battery powered by the cell will become inoperative.This object is accomplished not by a regulator circuit, but through theinherent electrical characteristics of the cell, which are therebypreserved even at low light levels.

For the purpose of the invention disclosed and claimed herein, I providea solar cell of otherwise conventional structure but having in additionto its front surface junction, a back surface junction that isdiscontinuous, i.e., one that has shorted portions rather than acontinuous layer of metal. Prior to alloying the metal into the backsurface of the wafer, it can be applied to the back surface in the formof stripes, spaced dots, or in other patterns, a variety of which willbe apparent; yet, it is the essence of the present invention that theentirety of the back junction should not be formed as a continuous zone,but that there be portions of the zone comprising the junction that havevarying properties in accordance with the pattern.

In general, I have found it preferable that aluminum or otherjunction-forming metal be applied to the back surface of the cell insuch a manner that less than one-half of such back surface is coveredwith metal. In this manner, the cell will not have the voltagegenerating capacity for example, of the cell described in my U.S. Pat.No. 3,990,097, but will generate sufficient voltage to trickle charge abattery and be substantially unaffected by substantial increases inlight impinging on the front surface of the cell. It will be apparent,of course, that the amount of back surface to which the junction-formingmetal will be applied will vary in accordance with the size of the celland the specific use to which the cell is to be put. Nevertheless,applying a layer of, e.g., aluminum so that less than one-half,preferably less than one-quarter of the area of the back surface iscovered, has been found suitable for most purposes.

With respect to the method of forming such a silicon solar energy cell,the junction-forming metal may be applied in an other-than-continuouspattern to the back surface of a cell in the same manner as it isapplied to the entirety of such back surface in my pending ApplicationSer. No. 705,063, which application is specifically incorporated byreference herein for its enabling disclosure. However, such method neednot be limited to that of my copending application. Thus, a pattern ofaluminum may, but need not be applied to a layer of diffusant glass onthe back surface of the wafer and then alloyed into the back surface ofthe cell to form a new junction. Instead, the back surface may beprotected from the diffusant and the aluminum applied in a pattern toit, then alloyed into the surface, or a diffusant glass may be formed onthe back surface, then removed by etching or other means and thealuminum applied thereto in a pattern. Alternatively, aluminum alloyingand diffusion may take place concurrently. Both the method ofapplication and the article produced according to my invention, however,will have less than the entirety of the back surface junction area ofthe cell composed of one type of junction-forming material.

These and other objects, features and advantages of photovoltaic cellsand methods of making them will become more apparent when considered inconnection with the following description of a specific embodiment andbest mode of my process and the product created by the practice thereof.That description will be more readily understood when viewed inconjunction with the accompanying drawing that forms a part of thisapplication and in which:

FIG. 1 is a diagrammatic illustration of a preferred method of making asolar cell according to my invention;

FIG. 2 is a bottom plan view of a most preferred embodiment of a solarenergy cell produced thereby;

FIG. 3 is a bottom plan view of an alternative embodiment of a solarenergy cell, and

FIG. 4 is a further enlarged fragmentary view showing a back junction ona greatly enlarged scale.

Referring first to FIG. 1 of the drawings, a boron-doped silicon wafer10 has a front surface 11 adapted for the impingement of light thereon,and a back surface 12. After diffusion at approximately 840° C, thefront surface 11 has a layer 13 of diffusant glass overlying it and theback surface 12 has a layer 14 of diffusant glass overlying it. Wherephosphorus is used as a diffusant in the normal manner, the glass layers13 and 14 will be formed from phosphorous glass. In addition, phosphorusboron junctions will be formed inwardly of the front and back surfacesof the wafer, the front surface junction being indicateddiagrammatically by broken line 16 and the back junction being indicatedby broken line 17. An aluminum coating is now applied to the phosphorglass 14 on the back surface of the wafer 10 through a mask, such maskusually being of a size larger than that of the wafer, such mask havingcircular apertures spaced uniformly therein. In the particular maskused, the apertures were 1 mm. in diameter, each circular aperture beingspaced from its most closely adjacent aperture by about 5 mm. The spaceoccupied by the apertures covered a small fraction of the entire area ofthe surface of the mask. Aluminum was applied through the apertures inthe mask, which was made of stainless steel, by evaporating aluminum ina vacuum system.

After the aluminum application, a layer of spaced aluminum dots wasformed on the phosphorous glass layer 14 on the back surface 12 of thewafer 10. The thickness of the dots of aluminum was about 5,000° A. Thenthe coated wafer was heated to 700° C, which was well in excess of thealloying temperature of aluminum, for 10 minutes, after which a junctionindicated by non-uniformly broken line 22 of polarity opposite to thatof junction 16 was formed at the back surface 12 of wafer 10, but onlyunder those areas where the aluminum had been deposited. In addition, alayer 23 overlying the back surface 12 of the wafer was formed. It wascomposed primarily of aluminum, oxygen and phosphorus. A back contactfor the silicon solar energy cell may then be formed by known methods.After a front contact has been applied, an operative silicon solarenergy cell was formed.

A detailed, fragmentary view of a diagrammatic illustration of a backportion of a wafer having a discontinuous junction formed therein isillustrated in FIG. 4 of the drawing. That illustration shows aluminumdots 26 that are adhered to and extend outwardly from a layer 23 ofphosphor glass which overlies the back surface 27 and is composed ofphosphorus, silicon and oxygen. However, at those portions of thephosphor glass layer 23 that underlie the circular aluminum areas 26,substantially cylindrical zones 23a are formed. Zones 23a are composedof doped phosphor glass, the dopant in this case being aluminum; thus,in this embodiment, zones 23a are composed of aluminum, phosphorus,silicon and oxygen. Extending inwardly from the back surface 27 of thewafer is a layer of silicon, doped with phosphorus and boron which layeris designated by reference numeral 24. Layer 24 constitutes a continuumin which certain zones underlying the aluminum dots 26 have been formed.These doped zones 24a are composed of aluminum, phosphorus, silicon andboron, and consist of spaced islands within thephosphorus-silicon-boron, and consist of spaced islands within thephosphorus-silicon-boron continuum 24. The doped portions 24a and thecontinuum 24 all terminate in a junction designated by reference numeral22. At those portions 22a of junction 22 that underlie the aluminumjunction-forming dots 26, an aluminum-boron junction is formed. At thoseportions of junction 22 that do not underlie aluminum dots 26 butunderlie continuum 24, a phosphorus-boron junction similar to the frontsurface junction is maintained. Since the phosphorus-boron portions ofjunction 22, designated by reference numeral 22b, is of the samepolarity as the front surface junction, that area of the junction willact in opposition to the front junction 16, the active part of the backjunction 22 being composed of spaced portions 22a that underlie thealuminum dots 26. Such spaced portions 22a enhance the function of thefront junction 16. In such enhancing portions 22a, the aluminumovercomes the polarity of the phosphorus-boron junction and forms analuminum boron junction of opposite polarity to the phosphorus-boronjunction. Consequently, in the specific cell that is utilized as apreferred embodiment herein, in cross section where there is an aluminumdot, may be referred to as an n-p-p+ cell, designating the junctionsfrom front to back of the cell, or as an n-p-n cell, where there is noaluminum dot. Wherein said P-p+ junction is considered to be a stepjunction.

A cell formed by the practice of the method of FIG. 1 is shown in bottomplan view of FIG. 2 of the drawings. In FIG. 2 a silicon solar energycell 25 is illustrated with uniformly spaced circular areas 26 ofaluminum on its back surface 27. After heating at 700° C the aluminumhas penetrated into the surrounding silicon and has formedaluminum-boron junctions through the phosphorous glass layer.

With respect to FIG. 3 of the drawing, the aluminum has been applieddirectly to the back surface 29 of a silicon slice 28 in the form ofstripes 30, although such stripes could also be applied to an overlyingphosphorous glass layer on the back surface 29. In the embodimentillustrated, the stripes have a width of approximately one-fifth of thedistance between adjoining stripes so that approximately 20% of the backsurface 29 of silicon wafer 28 will be covered by aluminum. Such stripesof aluminum can be applied by suitable masking techniques.

In practice, silicon solar energy cells having a back contact inaccordance with the method and product of my invention having been foundto have substantially constant voltage when exposed to light, whetherfrom natural or artificial sources, between 0.01 and multiple sunlevels. While one benefit that has been recognized from a cell of thestructure disclosed and claimed herein has been the feature ofsubstantially constant voltage, there are apparently other benefits aswell. From present indications, the solar energy cells according to thepresent invention will have improved efficiencies at low light levels.Other benefits will be apparent to those skilled in this art.

It will be understood that I have disclosed my invention herein byspecific recourse to preferred embodiments thereof only for the purposeof illustrating that invention, alterations and modifications of whichwill become obvious to those of skill in this art. For example, metalsother than aluminum may be used to form a back junction. In lieu of ametal alloy, a Schottky barrier or hetero-type junction may besubstituted in a manner known to those skilled in the art. As to allsuch obvious alterations and modifications, they are desired to beincluded within the purview of my invention, which is to be limited onlyby the scope, including equivalents, of the following appended claims.

I claim:
 1. A solar energy cell capable of maintaining a substantiallyconstant voltage, comprising a wafer having opposed front and backsurfaces, said front surface being adapted to absorb light impingingthereon, rectifying junction formed in said wafer at said front surfaceand another electrical junction formed in said wafer at said backsurface, said back junction being discontinuous and thereby formingportions thereof enhancing and opposing the action of the frontjunction, said enhancing portions in their entirety comprisingsubstantially less than the area of said wafer at said back junction,said enhancing portions forming a step junction with said wafer.
 2. Asolar energy cell as claimed in claim 1, in which said enhancingportions are uniformly spaced from each other by said opposing portions.3. A solar energy cell as claimed in claim 1 in which said wafer isformed from silicon and said enhancing portions are substantiallycircular.
 4. A solar energy cell as claimed in claim 1, in which saidwafer is formed from silicon and said enhancing portions are in the formof stripes extending from one edge of the wafer to the other, each ofsaid stripes being separated from adjoining stripes by a distance atleast equal to the width of the stripes.
 5. A solar energy cell asclaimed in claim 1, said back surface having contiguous therewith adiscontinuous electrically conductive coating comprising ajunction-forming metal, oxygen, silicon and an impurity.
 6. A solarenergy cell as claimed in claim 5, said cell further including a zonecomposed of aluminum and silicon extending into said wafer from saidback surface to said back junction.